This invention relates to a means for aligning, by optical means, one substantially planar object (hereafter called the mask) with respect to a second planar object (hereafter called the substrate). The invention achieves a high degree of sensitivity, accuracy and reliability through a novel design of alignment marks located on both the mask and the substrate.
In the practice of microlithography, such as in the manufacture of integrated circuits, it is often necessary to expose a pattern in a resist film on top of a preexisting pattern on the substrate. Generally, the latter pattern had itself utilized microlithography for its creation. This superposition of one pattern on top of another is generally referred to as "alignment". Often it is necessary to align the upper pattern on top of the lower pattern to within a small fraction (e.g. one fourth to one one-hundredth) of the minimum feature size achieved by the microlithography process. Thus, for example, if a microlithography process for fabricating integrated circuits is designed to yield a minimum linewidth of 1 micrometer, the requirement on alignment would typically be that critical pattern levels be aligned with respect to one another with a deviation from perfect superposition of less than 1/4 to 1/10 of a micrometer.
Many methods have been developed for the alignments of one level of microlithography to another. Typically, "alignment marks", such as crosses or rectangles, are created on the mask, and complementary marks are inscribed in the substrate. In proximity printing techniques, such as x-ray lithography, alignment is usually done by viewing both marks through an alignment microscope. In optical-projection lithography the images of the substrate alignment marks are sometimes projected, via a lens, onto the mask (generally, in this case, referred to as a reticle). Such schemes are sometimes referred to as "amplitude" alignment schemes.
In the mid 1970's, H. I. Smith and co-workers described an interferometric alignment scheme in which beams of light diffracted from gratings located on the mask and the substrate were made to interfere, and the intensity of this interference was interpreted to yield a measure of the alignment. This interferometric scheme is described in U.S. Pat. Nos. 4,200,395 and 4,340,305 granted to H. I. Smith, S. S. Austin and D. C. Flanders, incorporated by reference herein.
There were a number of shortcomings with this interferometric alignment scheme. Some were circumvented by subsequent developments in Japan, notably the use of two-frequency lasers and heterodyne techniques. However, the performance of heterodyne interferometric schemes, although able to detect misalignment of the order of 5 nm under carefully controlled experimental conditions, was deficient in more realistic experiments.
It is therefore an object of this invention to improve the performance that can be achieved with optical alignment systems under practical working conditions.